Method of measuring density properties of a particle distribution

ABSTRACT

A method and a device for measuring density properies of a particle distribution, in which method a first parameter relating to the particle distribution is measured at a first measuring point ( 302 ), at least part of the flow that passed through the first measuring point is guided to the second measuring point ( 304 ), a second parameter relating to the particle distribution is measured at a second measuring point ( 305 ), and said first and second parameters relating to the particle distribution are used to determine at least one density property of the particle distribution of the original flow ( 306 ).

FIELD OF THE INVENTION

The invention relates to a method and a device for measuring densityproperties of a particle distribution.

BACKGROUND OF THE INVENTION

With tightening environmental regulations, there is an increasing needfor the measurement of particle emissions. In particular, the need formeasurement is present in the development of filtering methods, in theresearch of various combustion processes, as well as in processes formonitoring actual emissions. One significant parameter in themeasurement of particle emissions is particle density. The particledensity is an important factor in a variety of properties which aresignificant in view of the particle being carried along, including forexample the settling velocity of the particle. For this reason, theparticle density is significant, for example, in the health effects ofthe particles, such as the accumulation of the particles in the lungs.

Problems involved in the measurement of the particle density aredescribed, for example, in the article by W. P. Kelly and P. H. McMurry,“Measurement of Particle Density by Intertial Classification ofDifferential Mobility Analyzer-Generated Monodisperse Aerosol” [AerosolScience and Technology 17: 199–212, 1992]. The same article also teachesa method of prior art for the measurement of particle density by meansof a DMA device (Differential Mobility Analyzer) and an impactor. FIG. 1shows the principle of operation of this method.

In the method disclosed in the article, a flow 13 a carrying a particledistribution to be analyzed is led to an apparatus 10 consisting of aDMA device 11 and an impactor 12. The flow is first led to the DMAdevice 11 which, by means of an electrical field, separates theparticles with a narrow electrical range of mobility from the flow to aflow 13 b to be led to the impactor 12. Particles whose electricalmobility is not within this narrow range are guided with flows 13 c and13 d away from the measuring device 10.

By means of the DMA device, it has thus been possible to separate themonodispersive aerosol flow 13 b having a given narrow electricalmobility distribution 14 b, from the polydispersive aerosol flow 13 ahaving an electrical distribution 14 a and being led to the measuringdevice.

This monodispersive aerosol flow is then led to the impactor 12 which,in a way known as such, classifies them on the basis of theiraero-dynamic diameter, collecting particles with different aerodynamicdiameters on different collection plates. By measuring the collectionplates, it is possible to determine the aerodynamic size distribution 15of the particles contained in the flow 13 b input in the impactor.

When both the electrical mobility diameter and the aerodynamic sizedistribution of the particle distribution to be analyzed are known, itis possible to compute the density of the particle distribution in theway presented in the article.

The above-presented solution of prior art involves the problem that thedensity can -only be determined for a narrow electrical mobility rangeat a time. In other words, by means of the method, the density can becomputed for the monodispersive flow 13 b by means of the DMA device. Todetermine the density distribution of the particles in thepolydispersive flow 13 a, this must be implemented, according to theabove-presented solution of prior art, by scanning, i.e. by firstdetermining the density in one electrical mobility range and thenchanging the adjustments of the DMA device in such a way that themeasurement is made in another electrical mobility range. This procedureis repeated until the density has been determined in the whole rangedesired.

For the above-presented scanning measurement to produce reliableresults, the flow 13 a to be analyzed should remain unchanged during thewhole measurement operation. Under real measuring conditions, there maybe temporal variations in the flow to be analyzed, for which reason theabove-presented solution of prior art is poorly suitable for thereal-time measurement of a flow containing polydispersive particlesunder real conditions.

SUMMARY OF THE INVENTION

It is an aim of the method described in the present application toeliminate the above-described problems of prior art and to provide asimpler method for determining the density properties of a particledistribution.

By means of the method and device of the invention, at least one densityproperty of the particle distribution is determined by measuring oneparameter related to the particle distribution at a first measuringpoint and another parameter related to the particle distribution at asecond measuring point. According to the invention, at least part of theflow that has passed the first measuring point is led to the secondmeasuring point. The measured parameters are used to determine at leastone density property of the particle distribution contained in theoriginal flow.

In an embodiment of the invention, the access of particles detected atthe first measuring point to the second measuring point is limited forexample by using, at the first measuring point, a measuring method whichremoves the detected particles from the flow to be analyzed. Thissimplifies the need for computing.

In another embodiment of the invention, one parameter is a parameterrelated to the mobility of the particles, and the other is a parameterrelated to their aerodynamic size. For these measurements, a mobilitychannel analyzer and an electrical low-pressure impactor can bepreferably used.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail withreference to the appended drawings, in which

FIG. 1 shows the solution of prior art for determining the density of aparticle distribution,

FIG. 2 shows an embodiment of the measuring device according to theinvention,

FIG. 3 shows a method according to the invention in a flow chart,

FIG. 4 shows another embodiment of the measuring device according to theinvention, and

FIG. 5 illustrates the relationships between the distributions detectedby different measuring devices.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 has been discussed above in connection with the description ofprior art.

FIG. 2 shows an embodiment of the measuring device according to theinvention. In this embodiment, the particle distribution to be analyzed,carried by a flow 13 a, is led to a first detector 21, in which at leastone parameter P1 relating to the determination of the density of theparticle distribution is measured from the particle distribution. Theparameter P1 preferably conveys information about the electrical ormechanical mobility of the particles.

The parameter P1, as well as the second parameter P2 to be describedbelow, can be not only single variables or other single values but alsoa given set of values or variables. Thus, for example, a set of threedifferent variables produced by a measurement at the measuring point, tofind out a parameter relating to a property of the particledistribution, can be considered one parameter in this context. In otherwords, said parameter P1 can preferably also be a set of parameters.

In an advantageous embodiment, the particles detected in a detector 21are either collected in the detector or separated from the flow by othermethods. The removal of the detected particles simplifies thecomputation to be made at a later step and makes it possible to executea more versatile computation, particularly in cases in which the entireflow 13 b to be led to the second detector 22 has passed through thefirst detector 21. In other words, it is advantageous in view of themethod according to the invention that the flow 13 b exiting the firstdetector 21 does not contain a significant quantity of such particleswhich were detected by the first detector 21.

The flow 13 b that has passed through the first detector 21 is led to asecond detector 22. Preferably, this takes place so that both measuringdevices are, at least for the parts required in the detection, installedinside the same structure that guides the flow, for example a duct. FIG.4 shows such an advantageous embodiment.

A part of the flow 13 b that has passed through the first detector 21can also be led past the second detector 22, if necessary. In FIG. 2,this is illustrated with a flow 13 e. The flow 13 f to be led to thesecond conductor 22 is preferably, for the particle distribution to beanalyzed, a representative sample of the flow 13 b that passed throughthe first detector 21. Corresponding flows are also drawn in FIG. 4.

The measurement according to the invention can be made even if the flow13 f to be led to the second detector 22 did not contain arepresentative sample of the flow 13 b coming from the first detector21, as long as it is possible to find out the differences between theparticle distribution contained in the flow 13 f to be led to the seconddetector 22 and the flow 13 a to be analyzed. Thus, the measurementaccording to the invention is also possible in a situation in which apart of the flow 13 a to be analyzed is guided past the first detector21 and mixed with the flow 13 b that passed the first detector 21,before the second detector 22. However, this kind of a situation is moredifficult to control by computing and makes the calibration of thedevice more complex.

The second detector 22 produces a second measuring signal P2 which canbe used to preferably determine the aerodynamic size distribution of theparticles contained in the flow 13 f led to the second detector 22. Thedensity properties of the particle distribution contained in the flow 13a to be analyzed can be determined by computing on the basis of themeasuring signals P1 and P2 from the first and second detectors,respectively.

In an advantageous embodiment, it is possible to compare the behaviourof the second signal P2 when the first detector 21 is turned on, with asituation in which the flow to be analyzed has free access to the seconddetector 22. On the basis of such a comparison, it is possible to findout the efficiency curve of the first detector 21. Such a solution makesit possible to use simpler and less expensive detectors, but on theother hand, it will make the device less suitable for real-timemeasurement.

According to an advantageous embodiment of the invention, the effectivedensity of the particle distribution to be analyzed can be computed bydetermining the median particle size according to the mobility size(D_(m)) as well as the median particle size according to the aerodynamicsize (D_(a)). When these factors are known, the effective density can becomputed from the following equation:D _(a) √{square root over (C _(a) ρ _(a) )} =D _(m) √{square root over(C _(m) ρ _(eff) )}

In the equation, the subindex a refers to the aerodynamic size and thesubindex m to the mobility size. C is Cunningham slip correction factor,ρ_(a) is the density corresponding to the aerodynamic size, i.e. unitdensity (1000 kg/m³), and ρ_(eff) is the effective density.

In an embodiment of the invention, the above-described first detector 21is selected so that the median particle size can be determined from thefirst signal P1 obtained, according to the mobility size (D_(m)), andthe second detector is selected so that the median particle size can bedetermined from the second signal according to the aerodynamic size(D_(a)). The Cunningham slip correction factors can be determined by anyway known as such for a person skilled in the art, for example by tablebooks. Thus, the only variable remaining unknown in the above equationis the effective density of the distribution to be analyzed, wherein itcan be solved.

FIG. 3 shows the flow chart of a method according to the invention,which implements the above-described determination of the effectivedensity. In the first step 301 of the method, the flow to be analyzed isled to a first measuring point where, in step 302, the first parameterrelating to the particle distribution is measured at the first measuringpoint. The parameter may be, for example, the magnitude 12 of anelectric current output by the detector, which can be used to evaluatethe median particle size according to the mobility size (D_(m)).

In step 303, the access of particles measured to the first measuringpoint to the second measuring point is restricted. This is preferablyachieved by using a collecting method of measuring, whereby the detectedparticles are removed from the flow under analysis in connection withthe detecting process.

In step 304, at least part of the flow that has passed through the firstmeasuring point is led to the second measuring point, where the secondparameter relating to the particle distribution is measured in step 305.The parameter may be, for example, the magnitude I1 of an electriccurrent output by the detector, which can be used to evaluate the medianparticle size according to the aerodynamic size (D_(m)).

In step 306, said first and second parameters relating to the particledistribution are used to determine at least one density property of theparticle distribution of the original flow. To determine the effectivedensity of the distribution, the above-presented formula can bepreferably used.

FIG. 4 shows an embodiment of the solution according to the invention.In the figure, the flow 13 a to be analyzed flows in a flue gas duct 49.The flow is first conducted through a mobility channel detector 41installed in the flue gas duct. At first, the flow 13 a passes a coronacharger 43 which charges the particles in the flow 13 a electrically.After this, the flow is introduced in an electric field E inducedbetween electrodes 43 a and 43 b. By the effect of the electric field E,the electrically charged particles are carried with their charge to theelectrode of the opposite sign. When hitting the electrode, the particleis discharged. This will cause a current I2 proportional to theelectrical mobility of the particle distribution to be analyzed.Preferably, at least a significant part of the particles collected atthe electrode are removed from the flow, for example by adhering to theelectrode.

Furthermore, the flue gas duct 49 is provided with a second detector 22which collects, in a way known as such, the particles that have passedthrough the mobility channel detector 41. When accumulating at thedetector, the particles generate a current I1. Preferably, the seconddetector may also be, for example, an electrical low-pressure impactorof prior art. Another advantageous alternative is to use an electricalprecipitator filter as the second detector. The advantage of theelectrical low-pressure impactor in comparison with the electricalprecipitator filter is, for example, the fact that the electricallow-pressure impactor can be used to measure the particle sizedistribution in real time, and for this reason, the above-describedmedian particle size distribution is easy to calculate.

The current signals I1 and I2 obtained from the detectors 41 and 22 areled to a separate computing unit 48 which uses them to produceinformation about at least one property of the particle distributioncontained in the original flow 13 a.

FIG. 5 shows the effect of the method according to the invention on thedetected particle distribution. If, in the device of FIG. 2 or 4, thesecond detector 22 is a detector measuring the particle sizedistribution, such as the above-mentioned electrical low-pressureimpactor, the detector can be used to produce the particle sizedistribution of FIG. 5, in which the aerodynamic particle size isindicated on the horizontal axis and the quantity of detected particlesis indicated on the vertical axis. If the first detector is notoperating but the flow to be analyzed is guided to the detector formeasuring the particle size distribution, the distribution according tothe envelope curve 51 indicated by a solid line in FIG. 5 is obtained.

When the first detector 21 in FIG. 2 or the mobility detector 41 in FIG.4 is started, the quantity of particles shown by the area limited by thedotted line 52 is removed from the particle size distribution detectedby the second detector. Thus, the second detector will detect thedistribution according to the broken line 53.

As stated above, it is not necessary that either of the used detectorswere capable of the actual computing of the particle size distribution.It is sufficient that the detectors produce a parameter relating to theparticle distribution, to be used in the computation of the desiredproperty. In such a situation, the first detector could produce a signalwhich is proportional to the particle distribution detected by the firstdetector. For example, the first detector could produce a current signalwhich is proportional to the vertically hatched area remaining below thedotted curve 52 in FIG. 5. Furthermore, the second detector couldproduce a current signal which is proportional to the horizontallyhatched area remaining below the broken line in FIG. 5.

In another embodiment, at least one of the measured parameters relatingto the particle distribution contains information about at least theshape of the particle distribution measured at the second measuringpoint. Such parameters include, for example, standard deviation or theabove-described median aerodynamic and mobility sizes.

By means of the above-described invention, the effective density of theparticle distribution contained in the particle flow to be analyzed canbe determined on the basis of simultaneous measurement in a large rangeof particle sizes. In other words, the present invention eliminates theneed of so-called scanning measurement according to prior art, byreplacing the classification step of prior art with the measuring step.This will make real-time measurement possible.

Hereinabove, some embodiments of the method and device according to theinvention have been described, but the invention is not restrictedsolely to these embodiments, but it can vary within the scope of theappended claims. In particular, it has been described above that thefirst detector is a mobility analyzer and the second detector is eitheran electrical precipitator filter or an electrical low-pressureimpactor. However, this arrangement is only presented as an example andit is intended to elucidate the principle of operation of the invention.In practice, under some conditions, it may be advantageous for examplethat the detectors are in a different order; thus, it is advantageouslypossible to measure the parameter relating to the aerodynamic size ofthe particles at the first measuring point and the parameter relating tothe mobility of the particles at the second point.

1. A method for measuring density properties of a particle distribution,in which method a particle flow to be analyzed is led to a firstmeasuring point, particles are detected at the first measuring pointwith the aid of an electrical field, a first parameter proportional tothe detected particles and relating to the particle distribution isproduced, at least part of the flow that has passed the first measuringpoint is led to a second measuring point, a second parameter relating tothe aerodynamic size of the particles is measured at the secondmeasuring point, and said first and second parameters are used todetermine at least one density property of the particle distribution ofthe original flow.
 2. The method according to claim 1, in which themethod is carried out in real time.
 3. The method according to claim 1,in which the access of particles detected at the first measuring pointto the second measuring point is restricted.
 4. The method according toclaim 3, in which the access of particles detected at the firstmeasuring point to the second measuring point is restricted by using acollecting method of measuring at the first measuring point.
 5. Themethod according to claim 1, in which at least the effective density ofthe particle distribution contained in the original flow is computed. 6.The method according to claim 1, in which at least one of said first andsecond parameter relates to the mobility of the particles.
 7. The methodaccording to claim 6, in which at least one of said first and secondparameter is used for computing the median mobility size.
 8. The methodaccording to claim 6, in which said parameter relating to the mobilityrelates to the electrical mobility of particles.
 9. The method accordingto claim 6, in which said parameter relating to the mobility relates tothe mechanical mobility of particles.
 10. The method according to claim1, in which at least one of said first and second parameters relating tothe particle distribution contains information about the shape of themeasured particle distribution.
 11. The method according to claim 1, inwhich said second parameter relating to the aerodynamic size of theparticles is used for computing the median aerodynamic particle size.12. The method according to claim 1, in which said second parameterrelating to the aerodynamic size of the particles is used for computingthe aerodynamic size distribution of the particle distribution.
 13. Themethod according to claim 1, in which at least one of said first andsecond parameters relating to the particle distribution is measured bymeans of a mobility channel detector.
 14. The method according to claim1, in which at least one of said first and second parameters relating tothe particle distribution is measured by means of an electricalprecipitator filter.
 15. The method according to claim 1, in which atleast one of said first and second parameters relating to the particledistribution is measured by means of an electrical low pressureimpactor.
 16. A device for measuring density properties of a particledistribution, comprising means for generating an electrical field at afirst measuring point, means for detecting particles at the firstmeasuring point by utilizing said electrical field, means for producinga parameter proportional to the detected particles and relating to themobility of the particles, means for measuring a second parameterrelating to the aerodynamic size of the particles at the secondmeasuring point, and means for computing at least one density propertyof the original particle distribution by means of said first parameterand a parameter relating to the aerodynamic size.
 17. The deviceaccording to claim 16, in which said means for measuring the parameterrelating to the mobility of the particle comprise a mobility channeldetector.
 18. The device according to claim 16, in which said means formeasuring the parameter relating to the aerodynamic size of the particlecomprise an electrical precipitator filter.
 19. The device according toclaim 16, in which said means for measuring the parameter relating tothe aerodynamic size of the particle comprise an electrical low-pressureimpactor.